The field of the invention is thickness sensors, and particularly sensors employed to measure the thickness of extruded webs.
To control the thickness of webs produced by an extrusion process a sensor capable of providing an accurate and reliable thickness feedback signal to the process control system must be provided. There are a number of such sensors available which employ a variety of technologies. These include: nuclear sensors which employ beta radiation; infrared sensors which employ electromagnetic waves at infrared frequencies; microwave sensors; laser sensors; x-ray sensors; and ultrasonic sensors. These sensors may operate by measuring the amount of radiation which is absorbed by the web, reflected by the web, or scattered by the web. No single sensor technology is suitable for all applications and the method which the sensor employs to make its measurement will usually depend on the nature of the web material and its thickness.